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Locking is a standard technique used in distributed computing and database systems to ensure data integrity by prohibiting concurrent conflicting updates on shared data objects. Internet-based collaborative systems are a special class of distributed applications which support human-to-human interaction and collaboration over the Internet. In this paper, a novel optional and responsive fine-grain locking scheme is proposed for consistency maintenance in Internet-based collaborative editors. In the proposed scheme, locking is made optional in the sense that a user may update any part of the document without necessarily requesting a lock. In the face of high communication latency in the Internet environment, responsive locking is achieved by granting the permit to the user for updating the data region immediately after issuing a locking request. Moreover, multiple fine-grain locks can be placed on different regions inside a document to allow concurrent and mutually exclusive editing on the same document. Protocols and algorithms for locking conflict resolution and consistency maintenance are devised to address special technical issues involved in optional and responsive fine-grain locking. The proposed locking scheme and supporting techniques were implemented in an Internet-based collaborative editor to demonstrate its feasibility and usability.